The motion of a moveable component (e.g., the direction and rate of movement of the moveable component) can often be characterized by means of an optical encoder. In the case of an absolute optical encoder, or an optical encoder that has been initially calibrated to a known position, an optical encoder can also be used to characterize the position of a moveable component.
Although optical encoders may take various forms, most can be characterized as linear or rotary. As their respective names imply, linear encoders are used to provide an indication of linear motion (and sometimes position), whereas rotary encoders are used to provide an indication of rotary motion (and sometimes position).
Most optical encoders can also be characterized as transmissive or reflective. In a transmissive optical encoder, a light source and a photodetector are positioned on opposite sides of a code member (e.g., a code strip or a code wheel). As the code member is moved by a movable component, a plurality of windows in the code member cause the photodetector to be illuminated with a varying pattern of light, which pattern can then be correlated with the motion of the moveable component. In a reflective optical encoder, a light source and a photodetector are positioned on the same side of a code member. Then, as the code member is moved by a moveable component, a plurality of reflectors on the code member causes the photodetector to be illuminated with a varying pattern of light.